An error that happens while the program is running. Even if the code is written with the proper syntax, there are things that can go wrong while the program is running. The program will be successfully compiled, it will begin to run, and as the program is running, the values being computed can cause the program to crash. This is a runtime error. If the user inputs a 0, and the program ends up dividing by 0 causing the program to crash, that is an example of a runtime error.